: Atrial fibrillation (AF) is the most common heart rhythm disorder: it affects more than two million Americans, is responsible for one-third of all strokes over the age of 65 years, and annually costs 9 billion dollars to manage. Furthermore, about 300,000 Americans die of sudden cardiac death annually, primarily due to ventricular rhythm disorders (ventricular tachycardia (VT and fibrillation) which result in intractable, extremely rapid heartbeats. Unfortunately, current pharmacological therapy for managing these disorders is often ineffective, thereby shifting emphasis to nonpharmacological therapy (e.g. ablation and pacing). Catheter ablation has been successful in managing many atrial and a few ventricular rhythm disorders. However, due to limitations in present mapping techniques, brief, chaotic, or complex rhythms such as AF and VT cannot be mapped adequately, resulting in their unsuccessful elimination. Advancing the management of abnormal heartbeats is contingent on developing mapping techniques that identify their mechanisms, localize their sites of origin, and elucidate effects of therapy. Our objective is to develop a catheter-based, cardiac electrophysiological imaging technique that simultaneously maps multiple endocardial electrograms on a beat-by-beat basis and combines three-dimensional activation-recovery sequences with endocardial anatomy. The hypothesis is that virtual electrical-anatomical imaging of the heart based on (1) cavitary electrograms that are measured with a noncontact, multielectorde probe and (2) three-dimensional endocardial anatomy that is determined with integrated, intracardiac echocardiography (ICE), provides an effective and efficient means to diagnose abnormal heartbeats and deliver therapy. Therefore, we will: (1) build a noncontact, electrical-anatomical imaging catheter-system that carries both a multielectrode catheter-probe for acquiring cavitary electrograms from multiple directions, and a central ICE catheter for acquiring endocardial anatomical images; (2) advance novel mathematical methods to compute endocardial electrograms and reconstruct three-dimensional activation-recovery sequences based on noncontact cavitary probe electrograms and geometry determined by ICE; and, (3) prove the utility of virtual electrical-anatomical imaging in the canine beating heart by characterizing models of AF, myocardial infarction, and VT and identifying their components, and by quantifying ablation lesions as assessed by both electrical and echocardiographic criteria. The proposed catheter can be introduced into the blood-filled cavity without surgery and provides three-dimensional electrical-anatomical images on a beat-by-beat basis. With this approach, one can pinpoint the site of origin and type of abnormal heartbeats and advance their therapy. In line with a Bioengineering Research Grant, the research develops a system the outcome of which is to improve the benefit-risk and benefit-cost relationships of patient care and advance heart rhythm-related research.